This invention relates generally to techniques for generating pulses and more specifically to techniques for converting arbitrary analog waveforms to produce sequences of pulses.
Ultra wide-band (UWB) is a fundamentally different information-transmission approach as compared to today""s continuous-wave RF-carrier signal transmissions. The UWB technology originated in the early 1960""s arising from studies on characterizing the microwave networks by their impulse response. A variety of names, including xe2x80x9cbaseband,xe2x80x9d xe2x80x9cimpulse,xe2x80x9d xe2x80x9cshort-pulse,xe2x80x9d and xe2x80x9ccarrier-free,xe2x80x9d identified the technology until the 1990""s, when the Department of Defense of the United States began using the term xe2x80x9cultra wide-band.xe2x80x9d
In UWB signaling, the transmission uses very short impulses of radio energy. This results in a characteristic spectrum that covers a wide range of radio frequencies. UWB systems have historically utilized impulse, or shock-excited, transmission techniques in which an ultra-short duration pulse (typically tens of picoseconds to a few nanoseconds in duration) is directly applied to an antenna which then radiates its characteristic impulse response. For this reason, UWB systems have often been referred to as xe2x80x9cimpulsexe2x80x9d radar or communications. In addition, since the excitation pulse is not a modulated or filtered waveform, such systems have also been termed xe2x80x9ccarrier-freexe2x80x9d in that no apparent carrier frequency is evident from the resulting RF spectrum. As the UWB signals have high bandwidth and frequency diversity, they are very well suited for various applications such as the wireless high speed data communication, etc. Typical UWB transmission systems include ON-OFF keying (binary amplitude shift keying ASK) and pulse position modulation (PPM).
To receive a signal that is originated from an ultra wide-band transmitter, an apparatus that is capable of triggering on very fast but low energy pulses is required. Two commonly used devices are the tunnel diode and the avalanche transistor. As the tunnel diode has a well defined i-v characteristic and its sensitivity is almost an order of magnitude better than that of the avalanche transistor, it is being used by most practitioners in the art.
In many developments of the ultra wide-band receivers, the tunnel diode has been used to detect the total energy in a pulse. In general, the tunnel diode is biased to operate as a bistable multivibrator as it has a characteristic of changing state whenever the charging carriers exceed a certain threshold.
In 1973, U.S. Pat. No. 3,755,696 introduced a constant false alarm rate (CFAR) circuit based on a tunnel diode detector. The circuit detects the noise dwells and data dwells to dynamically determine the optimum bias level of the tunnel diode that in turn improved the threshold sensitivity.
In 1994, U.S. Pat. No. 5,337,054 has shown a coherent processing method that is based on a tunnel diode detector that aims to improve the CFAR sensitivity. This is achieved by mixing the incoming signal with a continuous wave carrier that results in a beat frequency one-half of a RF (radio frequency) cycle for the given microwave burst. Thus, a monopolar baseband signal is obtained which maximizes the charge available to trigger the tunnel diode.
In 1999, U.S. Pat. No. 5,901,172 described a method that utilizes a microwave tunnel diode as a single pulse detector for the ultra wide-band applications. The optimum biasing point is determined only during the calibration phase at the system start-up. To gain good noise immunity, it uses an adaptive voltage variable attenuator that responds to the sample ambient noise.
Another type of UWB receiver uses the so-called xe2x80x9ccorrelatorxe2x80x9d concept. Correlator has proven to be the optimum detector for a narrowband communication system. However, it has yet to be shown that this concept is optimum for ultra wide-band communication. In the prior art implementations of this concept, a Pulse Position Modulation (PPM) technique is utilized. Information is sent out frame by frame. Within each frame, a pulse, whose width is much smaller than the time period of a frame, is uniquely positioned to represent a symbol. The correlator based receiver requires hundreds or thousands of these frames to gather enough energy to recover just one symbol.
In prior art solutions that use the tunnel diode as a detector, which operates in a bistable mode, there is a need to discharge the tunnel diode detector after each detection. Consequently, additional circuitry is required, and the speed of detection can be detrimentally limited by the time needed to discharge the tunnel diode.
In prior art solutions where a correlator detector is used to detect the UWB signal, hundreds or even thousands of frames are needed to recover one information symbol. This means the symbol rate will be much less than the rate at which the frames are transmitted.
Therefore, there is a need for a receiver whose symbol rate can be as fast as the rate the pulses are transmitted and not be bounded by any initialization requirement such as discharging a tunnel diode.
A method and apparatus for detecting a received ultra-wide band (UWB) signal includes receiving a transmitted UWB signal. In one embodiment of the invention, the transmitted UWB signal is an information waveform representative of one or more symbols to be communicated. The received signal is processed to produce a pulse waveform comprising groups of pulses. A detection waveform is applied to the pulse waveform to mask out extraneous pulse groups that do not correspond to the information waveform. A decoder is applied to the remaining groups of pulses to reproduce the original symbols.
A communication system is provided which incorporates the signaling method and apparatus of the present invention.